This invention relates to an improved polymerization process, and more particularly to a polymerization process wherein starting material such as a monomer, a mixture of monomers, a partially polymerized product, or a mixture of fully polymerized product and monomer or a monomer mixture is reacted with an initiator and a quaternary ammonium salt under polymerization conditions wherein polymerization or further polymerization of the starting material is initiated. The invention also relates to a polymerization process wherein an unsaturated polymer such as a polyester in a polymerizable monomer is reacted to form a crosslinked polymer structure.
Quaternary ammonium salts have been used previously in conjunction with potassium peroxydisulfate to initiate the polymerization of butyl acrylate in ethyl acetate solution. See, for example, J.K. Rasmussen, H.K. Smith, "Phase Transfer Free Radical Reactions: Polymerization of Acrylic Monomers in Organic Solution Using Potassium Peroxydisulfate in the Presence of Quaternary Ammonium Salts," Makromol. Chem., Volume 182, pages 701-703 (1981). However, in addition to using both an organic and an aqueous phase, these reactions require the use of a crown ether to make the peroxide soluble in the organic medium.
Potassium peroxydisulfate was also used as an initiator in the investigation of the kinetics of the phase-transfer-agent-aided free radical polymerization of styrene. This study was done at 60.degree. C. using tetrabutylammonium bromide as the phase-transfer agent and o-dichlorobenzene as a solvent. See, N.N. Ghosh and B.M. Mandal, Macromolecules, Volume 19, pages 19-25 (1986) .
In another study, a potassium persulfate-tetraethylammonium bromide initiating system was used for copolymerization of acrylonitrile in an aqueous ethylene glycol carbonate solution. A mixture of acrylonitrile, vinyl acetate and methylstyrene was used and the reactions were carried out at relatively high temperatures of 65, 70 and 80.degree. C. See, F. Sandescu and C.I. Simionescu, Acta Polymerica, Volume 37, pages 7-13 (1986).
Methyl methacrylate (MMA) has been polymerized employing dimethyl formamide solvent and using cetyl trimethyl ammonium bromide or cetyl benzyl dimethyl ammonium chloride with benzoyl peroxide as the initiating system. See, P. Ghosh and S.N. Maity, European Polymer Journal, Volume 14, pages 855-859 (1978); P. Ghosh and S.N. Maity, European Polymer Journal, Volume 16, pages 1115-1119 (1980). Both of these systems require the addition of dimethylformamide to the MMA to dissolve the quaternary ammonium salt. Moreover, the reaction is carried out at the elevated temperature of 40.degree. C.
MMA has also been polymerized at 50.degree. C. in the presence of trioctyl methyl ammonium salts initiated by dibenzoyl peroxide using a benzene or dimethyl phthalate solvent. See, A. Perdih, Polymer Bulletin, Volume 21, pages 151-158 (1989); A. Perdih, T. Malavasic and I. Vizovisek, Thermochimica Acta, Volume 134, pages 237-241 (1988).
In all of the above work, polymerization was carried out at temperatures above ambient temperature, a solvent was necessary or there were several phases. In contrast, the inventors have discovered a number of polymerization processes which can be carried out at room temperature, in the absence of an additional solvent, and/or in a single phase organic medium. In one form, selected quaternary ammonium salts in addition to an initiator that may be an organic peroxide, hydroperoxide or perester can be used to polymerize certain monomers or monomer mixtures. These selected quaternary ammonium salts plus a selected initiator can also be reacted with a partially polymerized compound in the syrup stage or a syrup prepared from a fully polymerized compound dissolved in a monomer or a mixture of monomers to form a solid polymer. In addition, the selected quaternary ammonium salt-initiator combination can be used to form a crosslinked polymer structure from an unsaturated polymer such as a polyester in a polymerizable monomer.
Solid polymers or viscous liquid polymers will often entrap solvent and it is difficult in certain cases, such as in thick castings, to remove the entrapped solvent. The present invention avoids this problem. Entrapped solvent can act like plasticizers and the removal of entrapped solvent will effect polymer volume.
It is, therefore, a principal object of this invention to provide improved polymerization processes which can be carried out without an additional solvent for the reactants.
It is a further object of this invention to provide improved polymerization processes which can be carried out at reduced temperatures.
It is yet another object of this invention to provide improved polymerization processes which can be carried out in a single phase organic medium.
It is still another object of this invention to provide improved polymerization processes for polymerizing selected monomers or monomer mixtures, or for forming polymers from partially or fully polymerized syrups easily and effectively.
Quaternary ammonium salt--peroxide/peroxy acid/hydroperoxide/perester systems of the type developed by the inventors have other advantages over previous systems for initiating polymerization as well. That is, the former are particularly well suited for casting polyester and methacrylate resins.
Thus, it is a further object of this invention to provide improved polymer resins and polymerization syrups.